This disclosure relates generally to the field of semiconductor assembly, and more specifically to a reflow method for lead-free solder for a semiconductor assembly.
A semiconductor assembly may include a circuitry on one component connected to circuitry on another component by a metal (or C4) layer comprising metallic interconnects made of solder. A semiconductor assembly may also include solder contacts to circuitry on a semiconductor wafer. The solder used for such interconnects and contacts has typically comprised metallic alloys containing lead (Pb), which have a relatively low melting temperature and are highly reliable. Unfortunately, lead is toxic and environmentally hazardous; therefore, lead-free solder is used as an alternative to lead-containing solder. Lead-free solder may comprise various alloying elements, such as tin (Sn) based alloys including, for example, silver (Ag) and/or copper (Cu). However, the solidification of lead-free solder may be difficult to control because solidification occurs in an undercooling state. Undercooling occurs when a metal is cooled without forming crystals to a temperature that is below the temperature at which crystallization normally takes place. Intermetallic compounds formed during the soldering process may be trapped at dendrite arms that form during undercooling, thus forming a relatively rigid solidified solder. Rigid interconnects may induce stress in the connected components, increasing the risk of mechanical failure or cracking of the semiconductor chip assembly, and rigid contacts may cause difficulties in connecting testing probes to the contacts during wafer testing, possibly resulting in multiple retests.